The term "pig", which is an acronym for pipeline inspection gauges, is used to refer to devices which are propelled along the interior of a pipeline for various purposes. A pig is typically propelled along the pipeline under the pressure of pipeline fluids, namely gas or liquids. In the absence of any speed control means for the pig, the pig will be propelled along the interior of the pipeline at the same speed as the pipeline fluids. The pig may tow other tools such as smart pigs that measure or monitor the condition of the interior of the pipeline, or it may stand alone and, for example, push debris along the pipeline and away from low spots in the pipeline. When used for any of the foregoing purposes, it is desirable that the pig maintain a preset speed in the range of, for example, 3-5 miles per hour (5-8 kilometres per hour).
U.S. Pat. No. 5,208,936 to Campbell describes a variable speed pig for pipelines having two plates with openings where the plates are disposed in the pig housing at opposite ends of flow through passageways. Annular seals seal off the annulus between the pig housing and the pipeline such that pipeline fluids cannot pass through this annulus. Openings in one plate can be aligned with the flow through passageways and with openings in the other plate through rotation of one plate relative to the passageways. Varying the degree of alignment of these openings in relation to the passageways, in turn, varies the amount of pipeline fluids which may pass through the flow through passageways and hence through the pig's housing with the result that the pig's speed can be increased or decreased relative to the pipeline fluid speed as desired.
U.S. Pat. No. 4,769,598 to Krieg et al. describes an apparatus for electromagnetically testing pipeline walls. The apparatus has two hollow cylindrical carriages which are coupled together for movement along the inside of a pipeline. The carriages are of a diameter which is approximately half of the pipeline's diameter and are supported against the pipeline walls by means of rollers disposed around the circumference of the carriages. Two sealing discs are mounted near the front of the first carriage which bear with their circumference against the pipeline wall and shut off the cross-section in an outer annular region at the front end of the first carriage. Between the two sealing discs and a reduced area nose of the first carriage is an annular passage through which pipeline fluids can pass from behind the apparatus to the front. Two perforated discs are mounted near the front of the first carriage and are rotatable in relation to one another. The degree of alignment of the perforations in the two discs permits the speed of the apparatus to be varied by permitting more or less pipeline fluid to pass through the annular passage. A set of brakes is also mounted on the carriage for emergency braking or if the control capabilities of the annular passage are exceeded.
The pigs described in the foregoing Campbell and Krieg patents are limited by the extent to which they are capable of permitting the bypass of pipeline fluid (i.e. through the pig) so as to vary the speed of the pigs. There has therefore developed a need for a pig that is better able to maintain a speed within a preset range in a pipeline, while bypassing any excess pipeline fluids not required for propulsion of the pig.